Core electron excitations in U⁴⁺: modelling of the <em>n</em>d¹⁰5f² → <em>n</em>d⁹5f³ transitions with n = 3, 4 and 5 by ligand field tools and density functional theory

Ligand field density functional theory (LFDFT) calculations have been used to model the uranium M4,5, N4,5 and O4,5-edge X-ray absorption near edge structure (XANES) in UO₂, characterized by the promotion of one electron from the core and the semi-core 3d, 4d and 5d orbitals of U⁴⁺ to the valence 5f. The model describes the procedure to resolve non-empirically the multiplet energy levels originating from the two-open-shell system with d and f electrons and to calculate the oscillator strengths corresponding to the dipole allowed d¹⁰f² → d⁹f³ transitions appropriate to represent the d electron excitation process. In the first step, the energy and UO₂ unit-cell volume corresponding to the minimum structures are determined using the Hubbard model (DFT+U) approach. The model of the optical properties due to the uranium nd¹⁰5f² → nd⁹5f³ transitions, with n = 3, 4 and 5, has been tackled by means of electronic structure calculations based on the ligand field concept emulating the Slater–Condon integrals, the spin–orbit coupling constants and the parameters of the ligand field potential needed by the ligand field Hamiltonian from Density Functional Theory. A deep-rooted theoretical procedure using the LFDFT approach has been established for actinide-bearing systems that can be valuable to compute targeted results, such as spectroscopic details at the electronic scale. As a case study, uranium dioxide has been considered because it is a nuclear fuel material, and both atomic and electronic structure calculations are indispensable for a deeper understanding of irradiation driven microstructural changes occurring in this material

Chemical state and atomic scale environment of nickel in the corrosion layer of irradiated Zircaloy-2 at a burn-up around 45 MWd/kg

Zircaloy-2 is used as fuel cladding in commercial boiling water reactors (BWR). A limiting factor for fuel longevity is the waterside corrosion of the cladding during in-service reactor operation and associated hydrogen pickup to the alloy. It is well known that the alloying elements (such as Cr, Fe, Ni etc.) including intermetallic precipitates (also termed as SPP) distribution influences both the oxidation process and hydrogen uptake evolution in this material. This paper reports an experimental investigation on the atomic scale microstructure of nickel-containing intermetallic particles with an emphasis on the oxidation and nickel-dissolution of SPP, and a combined experimental and computational study of solute nickel located in the corroded zirconium oxide microstructure. An irradiated cladding sample, taken from a BWR fuel rod, was prepared for the analysis using electron probe microanalysis (EPMA) and synchrotron-based micro-beam X-ray techniques (μXRF, μXRD and μXAS). The results show that the Ni-bearing SPP in the oxide layer are neither fully dissolved nor entirely oxidized at the given burn-up of the sample investigated. Conversely, all solute nickel present in the corroded layer is mostly oxidized and has an apparent homogeneous Ni2+distribution. By analyzing the μXAS spectra measured at the Ni absorption edge, we have obtained quantitative structural information about both irradiated SPP and the Ni coordination environment in the corrosion layer. There exists strong structural disorder in intermetallic Ni-bearing SPP as also evidenced by μXRD study. The basic structure away from SPP in the oxide area is composed of oxidized nickel atoms adjacent to oxygen vacancies. Finally, first-principles density functional theory (DFT) calculations have been used to discern the nickel speciation in zirconium oxide microstructure that is complementary to the multitude of experimental information. From this joint theoretical and experimental approach, significant insights into the structural specificity of Ni2+ions in monoclinic ZrO2, electronic factors governing the electron transport processes in the corrosion layer, and the apparent influence of nickel on the hydrogen ingress behavior in Zircaloy-2 are obtained

Nuclear material investigations by advanced analytical techniques

Advanced analytical techniques have been used to characterize nuclear materials at the Paul Scherrer Institute during the last decade. The analysed materials ranged from reactor pressure vessel (RPV) steels, Zircaloy claddings to fuel samples. The processes studied included copper cluster build up in RPV steels, corrosion, mechanical and irradiation damage behaviour of PWR and BWR cladding materials as well as fuel defect development. The used advanced techniques included muon spin resonance spectroscopy for zirconium alloy defect characterization while fuel element materials were analysed by techniques derived from neutron and X-ray scattering and absorption spectroscopy